Biosynthesis of ferredoxin-NADP+ oxidoreductase. Evidence for the formation of a functional preholoenzyme in the cytoplasmic compartment.

Biosynthesis of ferredoxin-NADP+ reductase in higher plants was investigated in relation with the mechanism of formation of the holoenzyme. The putative precursor of the flavoprotein, obtained after cell-free translation on a wheat germ extract primed with poly(A)-rich mRNA, was able to spontaneously bind free FAD, rendering a functional prereductase. The newly synthesized preholoenzyme showed diaphorase and cytochrome c reductase activities, an apparent molecular mass of 45 kDa, and contained FAD as the only flavin cofactor. It gave a positive reaction towards antisera against mature ferredoxin-NADP+ reductase. On the other hand, intracellular distribution of flavin-synthesizing enzymes indicates that FAD formation occurs in the cytoplasm; that is, in the same compartment as the site of reductase synthesis. On the basis of the preceding data a model is presented for the biosynthesis of the enzyme in vivo, involving conjugation of the apoprotein with FAD in the cytoplasm, followed by transport of the preholoreductase across the chloroplast envelope to reach its final destiny in the thylakoid membrane.